If I'd get almost the same that Barca promised me at another club that actually wants me and plays me in my favourite position I probably would. I don't know the numbers though. If what Barca promised him is much higher than what I'd get anywhere else I'd probably stay too. Hard to tell.
What I don't get: I was of the impression Barca already OWE him some money? Did that turn out not to be correct then?
Morally : 50/50
Legally : No
Let's say, you're on 150k / week. But due to COVID, I can't pay you 150k / week, instead I'm offering you 50k/week raising incrementally to 100k/week on 2nd year, and say 300k/week on 3rd year and probably let's assume 400k on the 4th / 5th year. CBA to do the actual math. But it's a whole new contract. So instead of a wage cut, you're getting your wage 2 years from now, which is fair enough considering it's COVID time.
You sign it, knowing that you love where you are, and understanding that it's COVID.
Now on your 2nd year, where next year you'd be on 300k/week. Would you move? And if you move? are you owed anything
legally? The answer is probably no, if you move at your own will, it's not Barcelona's fault, they're honoring their deal. Its you who wants to move. Not their fault. You will still get your loyalty bonus (however much that is) because you didn't put a transfer request, and the club accepted a bid for you in good manner. You're not forcing your way out, neither the club forcing your way out.
Are you legally entitled to whatever the money you think you're own? Nope. You decided to sign the new contract willingly, with no guns in your head.